Starting at level 50 and going on every five levels after that (another cap at 55, 60, 65, etc) there are quests known collectively as Limit Breaks or Genkais which you need to complete in order to be allowed to level past that level cap limit.

You need to visit the NPC Maat in Ru'lude Gardens, who will give you the "In Defiant Challenge" quest, asking you to get an Exoray Mold, a Bomb Coal, and an Ancient Papyrus. These items are Rare/ex, so you'll need to farm them up off the mobs that drop them in Crawlers' Nest, Garlaige Citadel, and the Eldieme Necropolis (respectively).